4 Major Virus Symptoms
Your system crashes or locks up often.
It randomly restarts.
You see strange error messages or pop-up boxes especially alerts about firewalls or viruses.
Your system suddenly runs much slower, and the key here is suddenly. If it progressively slows down, it's probably getting bogged down by too many programs running in the background, you haven't defragmented it recently (or ever), or you are nearing full capacity on the hard drive. But if the computer suddenly takes minutes to load a web page, or suddenly takes 10 minutes to shutdown/start-up it's a good clue that you have a virus.
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